1. Field of the Invention
The present invention relates to a vaporizer for vaporizing a liquefied petroleum gas (LPG) such as liquid propane gas or liquid butane gas, or a liquefied natural gas (LNG). More particularly, the invention relates to an air-heated/hot-water type vaporizer applicable in a cold district, in a cold climate caused by abnormal weather, or in the coldest season, and a gas producing plant using such a vaporizer. The present invention can suitably be used for the production of propane gas, butane gas or a natural gas mixed with air, respectively, from liquid propane gas, liquid butane gas or liquefied natural gas.
2. Description of the Related Art
As is disclosed in Japanese Patent Application Laid-Open No. 61-180099, the present applicant proposed a plant for the production of 13A propane-air gas by vaporizing liquid propane gas in an air-heated type forced vaporizer and mixing with air.
In the air-heated type forced vaporizer used in this 13A propane-air gas producing plant, the heat exchanger is prepared with a finned heat transfer tube (multiple-unit heat transfer coefficient: 25.3 kg/m.sup.3.hr..degree. C.) made of aluminum having a high heat transfer efficiency similar to that of liquid oxygen or liquid nitrogen, and the boiling point of liquid propane gas flowing into the vaporizer is reduced by reducing the pressure at the liquid inlet of the vaporizer, for example, to -27.degree. C. with 1 kg/cm.sup.2.G to achieve a temperature difference from the open air temperature, thereby conducting gasification by means of the finned heat transfer tube.
Since the air-heated type forced vaporizer uses the open air temperature as a heat source, and vaporizes liquid propane gas under the effect of a temperature difference between the open air temperature and the liquid temperature of liquid propane gas, this 13A propane-air gas producing plant does not require an initial cost for the heat source facilities for vaporization or a running cost of these facilities, thus providing a feature of permitting production and supply of town gas at a very low cost.
In the aforementioned 13A propane-air gas producing plant proposed by the present applicant, however, the standard design capacity of the air-heated type forced vaporizer is continuous operation for four hours under conditions including an open air temperature of at least -8.degree. C., cloudy weather and windless state, taking account of the initial cost of facilities. It therefore becomes difficult or even impossible to use in a coldest district or during the coldest season.
In a district located in the south of Kanto, Japan, for example, conditions can well be coped with by the air-heated type forced vaporizer of the above-mentioned design specifications, but vaporizing ability is reduced in other district or in winter. According to the result of experimental studies carried out by the present applicant. The northern limit is the northern district of Kanto.
Even in a district south of Kanto, severe frosting onto the vaporizer caused as a result of continuous operation of the plant necessitates a spare air-heated type forced vaporizer for switching, thus leading to a higher initial cost.
Installation of a separate hot-water type vaporizer to cope with a cold district or the coldest season would result in a further higher initial cost.
In Japan, it is tried to produce butane-air gas from liquid butane gas as raw material in a town gas producing plant.
Liquid butane used in the butane-air gas plant has such a high boiling point as about 15.degree. C. at 1 kg/cm.sup.2.G and must be vaporized by means of a vaporizer using electricity, hot water or steam as a heat source, so that it has been impossible to use a conventional air-heated type forced vaporizer as that used in the aforementioned 13A propane-gas producing plant.
In view of the problems as described above, the present applicant proposed an air-heated/hot-water type vaporizer, in Japanese Patent Application Publications Nos. 6-89879 and 6-89880, air-heated/hot-water type vaporizers permitting switch operation in response to weather and other conditions: air-heated type in summer and hot-water type in winter, in general, and satisfactory vaporization of LPG even in a cold district or during the coldest season.
The air-heated/hot-water type vaporizer disclosed in Japanese Patent Application Publication No. 6-89879 has a supply-side manifold, a discharge-side manifold, and a plurality of heat transfer tubes connecting the supply-side manifold and the discharge-side manifold. The supply-side manifold has a double-pipe structure comprising a liquefied petroleum gas tube and jacket tubes arranged so as to surround the outside of this liquefied petroleum gas tube. Liquefied petroleum gas is supplied to the liquefied petroleum gas tube, and hot water is supplied to the jacket tubes.
The air-heated/hot-water type vaporizer disclosed in Japanese Patent Application Publication No. 6-89880 has a configuration similar to that of the air-heated/hot-water type vaporizer disclosed in Japanese Patent Application Publication No. 6-89879 except that a heating tube is spirally wound round the outer periphery of the supply-side manifold, and hot water is supplied into the heating tube.
An air-heated/hot-water type vaporizer of such a configuration permitted increase in the vaporizer and vaporization capacities, and efficient removal of frost depositing onto the vaporizer as a result of continuous operation and was efficiently applicable to a 13A propane/air gas producing plant or a butane/air gas producing process without need of a spare vaporizer for switching.
As disclosed in Japanese Unexamined Patent Application Laid-Open Nos. 6-94194, 9-287697 and 9-287698, the present applicant proposed an air-heated/steam type vaporizer using steam in place of hot water for the purpose of improving the aforementioned air-heated/hot-water type vaporizer.
As a result of further study and experiments carried out by the present applicant, the air-heated/steam type vaporizer was found to have a higher thermal conductivity as compared with the air-heated/hot-water type vaporizer, and permits improvement of the vaporizer and vaporization capacities, but has problems in the necessity of maintenance control and water control of the boiler and a high running cost.
As compared with the air-heated/steam type vaporizer, the airheated/hot-water type vaporizer was found to provide the following advantages:
(1) Use of hot water at a temperature of from 20.degree. C. to 80.degree. C. in place of steam results in low in-boiler temperature and pressure, thus leading to almost complete absence of deposited scale and a longer service life.
(2) A water softening apparatus for supply water is not necessary. This corresponds to a lower equipment cost. A boiler compound for water control is not needed, leading to a lower running cost.
(3) A simple structure results in fewer failures.
(4) It is easier to effect operational control and handling.